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A detailed evaluation of two typical bridge configurations is carried out to assess the suitability of linear response spectrum analysis to combined horizontal and vertical response spectra for predicting moment demands in the bridge girder.The process consists of first selecting the corresponding Caltrans ARS curve based on the earthquake magnitude and peak rock acceleration at the bridge site.This spectrum is applied in the longitudinal direction of the bridge.The vertical spectrum is built up using the procedures described in the report by Kunnath et al.(2008).The moment demands of the bridge girder are then computed from elastic response spectrum analysis considering sufficient number of modes using the longitudinal and vertical spectra.These results are compared to demands estimated with time history simulations using nonlinear models of the bridge columns.It is concluded that response history analysis using the horizontal and vertical design spectra is a valid preliminary approach to estimate the effects of vertical ground motions on ordinary highway bridges.The estimates from response spectrum analysis are typically more conservative for multi-span bridges than for 2-span overcrossings.